The fact that all A/J mice produce anti-p-azophenylarsonate (anti- Ar) antibodies, some of which share cross-reactive idiotypic specificity (CRI), made it possible to suppress the appearance of the idiotype in any A/J mouse by using an antiidiotypic antiserum prepared against the anti-Ar antibodies of another mouse. This in turn has made feasible a number of related studies. We now plan to ascertain whether suppression is an active process, mediated by T cells, or whether it simply reflects deletion or inactivation of the clone of cells producing CRI. T and B cells will be isolated from "hyperimmune, suppressed" cell populations to ascertain whether suppression is mediated by suppressor T cells or, through competition for antigen, by B cells. By studying the reemergence of CRI as a function of time after administration of antiidiotypic antiserum we should obtain information concerning the rate of generation of cells bearing a particular receptor. Investigations will continue of competititon for antigen by clones of cells possessing and lacking receptors bearing CRI. These will be carried out through adoptive transfers of mixtures of the two classes of cells.